Recently, environment preservation measures, beginning with measures against global warming, are becoming unavoidable issues to be promoted for advanced societies. As part of these measures, environmentally friendly lubricating oils are required to satisfy severer conditions. These lubricating oils, in particular those for vehicles, are required to more efficiently reduce fuel consumption in order to abate exhaust carbon dioxide emissions.
Under these situations, a variety of lubricating oil compositions have been proposed to develop fuel-efficient lubricating oils. Many of these compositions are incorporated with various friction reducing agents, e.g., those of molybdenum compounds (“Mo compounds”). They are intended to reduce fuel consumption by reducing friction at slide members. For example, Patent Document 1 (JP-A 6-313183) applied by the inventors of the present invention discloses that friction can be further reduced by incorporating a Mo compound, e.g., molybdenum dithiocarbamate (MODTC) or molybdenum dithiophosphate (MoDTP) in a base oil having specific properties. Patent Document 2 (JP-A 6-336592) discloses that a combination of MoDTC and a specific additive reduce friction still more efficiently.
Friction reducing agents, e.g., those containing a varying Mo compound or the like, exhibit their functions/effects in mixed to boundary lubrication conditions. However, fluid lubrication is predominant in some lubricated members in vehicles. For these members, reducing lubricating oil viscosity is effective for reducing fuel consumption, in particular under high share rates. Effective temperature for fuel saving is in a range of about 80 to 100° C.
However, lubricating oil viscosity decreases as temperature increases. Therefore, a lubricating oil having an excessively low viscosity at 80 to 100° C. will cause troubles related to wear resistance at high temperature, because of broken oil film. As a result, the viscosity standards under high temperature/high shear rate conditions at 150° C. (HTHS150° C. viscosity) are set down for engine oil quality management. According to the viscosity grade standards SAEJ300, SAE20 oil as the lowest viscosity grade is required to have an HTHS150° C. viscosity of 2.6 mPa·s or more. In other words, the HTHS150° C. viscosity standards provide restrictions on conventional techniques to reduce shear viscosity in an intermediate temperature range of 80 to 100° C.
It is essential for a fuel-efficient lubricating oil composition to keep viscosity at a given level under high temperature/high shear rate conditions for securing wear resistance characteristics and, at the same time, to reduce shear viscosity in an intermediate temperature range from 80 to 100° C., an effective temperature range for reducing fuel consumption.
Techniques of prior art are reviewed from the above viewpoint. For example, Patent Document 3 (JP-A 2001-181664) proposes an engine oil having fuel-efficient and low-viscosity characteristics, comprising a base oil incorporated with a viscosity index improver of polymethacrylate, where the base oil has specified properties of viscosity index, aromatic content and so forth. However, it merely presents compositional relationships defining lack of fuel-efficient or low-viscosity characteristics when the base oil fails to satisfy specified properties, composition viscosity deviates from a specified range, or a viscosity index improver of olefin copolymer is used. It is silent on control of shear viscosity in an intermediate temperature range from 80 to 100° C., although discussing HTHS150° C. viscosity in the preferred embodiments. Therefore, there is room for further reduction of fuel consumption. Patent Document 4 (JP-A 2002-12884) discloses a base oil and 6 species of additives for reducing fuel consumption while satisfying cleanness and wear preventing characteristics. However, it merely presents compositional relationships defining lack of one of the above characteristics when a component or its content deviates from a specified range. It is silent on a fuel reduction effect brought by controlling shear viscosity in a range from 80 to 100° C. while keeping a viscosity under high temperature/high shear rate conditions.
As described above, the prior art techniques have neither disclosed nor suggested a fuel-efficient lubricating oil composition which has a reduced shear viscosity in an intermediate temperature range from 80 to 100° C. to reduce fuel consumption while keeping a viscosity at a given level under high temperature/high shear rate conditions.